Hi, just found a solution in these same forums after diferent searches in google that let me to here, I will paste the solution, think it was in the Nook forum:

Answear to the question:
What you can try, is specifying the following in the stylesheet:
Code:

ol.loweralpha {
list-style-type: lower-alpha;
}

and then use <ol style="loweralpha"> instead of type.

Answear to the solution:
I included the code for ol.loweralpha in my stylesheet as directed, and when I replaced <ol type="a"> with <ol class="loweralpha">, the lettered lists displayed perfectly on the NOOK Color and on the iPad -- a great solution.